I left my bags a lot, for a long time. They would sit in an untidy pile, or in a tidy basket, by the front door "so I won't forget them when I go shopping." I would forget. I have found I have to make it more inescapable than that. I finally realized that I have to take them out to the car, for storage, every time.
Simplifying the steps to remembering my bags was a major step in remembering them. If I forgot to bring them into the store, I either ran back out to the car or I loaded the groceries into my bags at the car. The little bit of hassle helped me learn the new habit. At this point I'm ready to join the "bring your own bag" Olympic team -- I bring cloth bags for produce and bulk items.(for the record, they store food really, really well and I no longer have mushrooms turn to slime before I get a chance to use them. I can tell you how it works if you're interested).
I really like cloth bags for a number of reasons. I appreciate their environmental benefits. I appreciate the five cent refund from a number of retailers. (I would turn in a coupon for 45 cents, so why not get back five cents a bag, every trip?) But I really appreciate not having plastic grocery bags taking over my cupboards!
I know lots of people find them useful, but they mostly just went to live in the cupboard under my sink, where they leaped out every time I tried to wash the dishes. I tried to keep them in a special sock, or an old tissue box, but they always overwhelmed the box or bag and escaped across the floor.
Now, I use cloth bags for shopping, and empty bread bags for picking up Things I Don't Want To Touch. When I have too many bread bags to fit in my container, I thin the heard and put the extras out in the car, next to my shopping bags, so I can recycle them
Note: You don't have to use the ugly bags stores sell at the register. I have a pretty floral bag I never used for anything else, and an old beach bag that never went to the beach. And if you find you have more bags than you every use, offer them to the person behind you at the store who forgot hers. Only keep what you need, and your car won't be overcluttered with shopping bags, either.